1. Field of the Invention
The invention relates to a circuit for reproducing data, and an apparatus for reading data out of a magnetic disc, and more particularly to such a circuit and such an apparatus both of which is capable of solving a problem that there occurs reproduction error due to fluctuation in a reproduction voltage while data is being reproduced by means of a magneto-resistance effect type head.
2. Description of the Related Art
A magneto-resistance effect type head is mounted on an apparatus for magnetically reproducing data, as a reproduction head. A magneto-resistance effect type head operates under such a principle as mentioned below. If a magnetization orientation varies, an electrical resistance of a magneto-resistance effect element accomplishing electromagnetic conversion also varies. Hence, by keeping a constant sense current to run through the magneto-resistance effect element, variation in a magnetization orientation in the magneto-resistance effect element can be detected as variation in a voltage.
An electrical resistance of a magneto-resistance effect element also varies, if a temperature of the magneto-resistance effect element varies. That is, a voltage for reproducing data varies not only by variation in a magnetization orientation, but also by variation in a temperature of a magneto-resistance effect element.
A temperature of a magneto-resistance effect element varies, for instance, when there is generated frictional heat because of contact between a magnetic recording medium and a magneto-resistance effect element, or when frictional heat generated due to contact between a magnetic recording medium and parts other than a magneto-resistance effect element transfers to a magneto-resistance effect element.
If a temperature of a magneto-resistance effect element varies, a voltage for reproducing data varies, resulting in that signals other than signals transmitted from a magnetic recording medium and indicative of a magnetic field profile are reflected on a waveform of a reproduced wave, which would cause reproduction errors.
A voltage used when a magnetic field is not applied to a magneto-resistance effect element from a magnetic recording medium is referred to as a base line. Since no signals are received in a magneto-resistance effect element, a voltage for reproducing data is kept constant. However, if a temperature of a magneto-resistance effect element varies, a voltage for reproducing data also varies even in a magneto-resistance effect element receiving no signals, resulting in fluctuation in a base line.
When a base line varies due to such fluctuation in a temperature, there has been conventionally employed a high-pass filter to compensate for such distortion, because a frequency of fluctuation in a base line is small.
A high-pass filter having a cut-off frequency which is as high as possible in the range that the cut-off frequency is smaller than a minimum recording frequency could have greater effect for suppressing fluctuation in a base line.
However, a high-pass filter having an excessively high cut-off frequency would be accompanied with a problem. The first reason is that a higher cut-off frequency would cause a reproduced wave to be distorted to a greater degree. The second and main reason is that since a cut-off frequency at which a reproduced wave is much distorted is dependent on a rotational frequency of a magnetic recording medium, a rotation radius (track location) of a magnetic recording medium on which a magnetic head makes slide movement, and so on, a cut-off frequency cannot be defined in a single value.
An optimal cut-off frequency has been conventionally determined in trial and error. However, as mentioned earlier, if a cut-off frequency is to be determined in trial and error, a cut-off frequency has to be altered each time a rotational frequency or a rotation radius of a magnetic recording medium varies. In addition, a cut-off frequency has to be altered in dependence on a tendency of fluctuation in a base line.
As mentioned earlier, a cut-off frequency of a high-pass filter used for suppressing fluctuation in a base line cannot be defined into a single frequency, and it is impossible to suppress base line fluctuation having a frequency higher than a cut-off frequency.
Hence, it is desired that a cut-off frequency is set as high as possible. However, if a cut-off frequency is set higher than a minimum frequency of a component of a signal transmitted from a magnetic recording medium, a reproduced signal would be removed, and/or a reproduced wave might be distorted.
Japanese Unexamined Utility Model Publication No. 5-85060 has suggested a circuit for processing signals, including a magneto-resistance effect type element. The circuit further includes a high-pass filter for compensating for a temperature characteristic of the magneto-resistance effect type element.
However, the Publication does not refer to a cut-off frequency of the high-pass filter, and hence, does not mention a problem of fluctuation in a base line.
Japanese Unexamined Patent Publication No. 8-87707 has suggested an apparatus for magnetically recording and reproducing data. The apparatus is comprised of a circuit for generating a rectangular wave signal in accordance with magnetization inversion occurring on a magnetic recording medium, and a subtracter for subtracting the thus generated rectangular wave signal from a reproduced signal to thereby remove base line shift noise. According to the apparatus, it is possible to remove base line shift noise generated by MR head, and thereby, reproduce data without noise.